docs: bump stable version to v2.15.1 (#14927)

This PR was automatically created by the [release
script](https://github.com/coder/coder/blob/main/scripts/release.sh).

Please review the changes and merge if they look good and the release is
complete.

You can follow the release progress
[here](https://github.com/coder/coder/actions/workflows/release.yaml)
and view the published release
[here](https://github.com/coder/coder/releases/tag/v2.15.1) (once
complete).

docs/install/kubernetes.md

@@ -145,7 +145,7 @@ locally in order to log in and manage templates.
    helm install coder coder-v2/coder \
        --namespace coder \
        --values values.yaml \
-       --version 2.15.0
+       --version 2.15.1
    ```
 
    You can watch Coder start up by running `kubectl get pods -n coder`. Once